Plasma sources excited by radiofrequency (RF) in a frequency range of between 1 MHz and 20 MHz and comprising a plasma space which is closed off by a grid and from which a plasma beam is extracted are known, wherein a distinction is made between inductive and capacitive excitation of the plasma to which a magnetic field is usually applied. In the case of such radiofrequency plasma sources having inductive and mixed inductive/capacitive excitations, modified Helmholtz coils are used which generate substantially homogeneous fields that are perpendicular to an induction RF coupling-in coil or turn and lead to an increase in the plasma density.
DE 694 210 33 T2 discloses, for example, an inductive plasma source which is operated in the radiofrequency range (RF) and in which, with a reduced number of system components, the plasma density is increased by permanent magnets arranged outside a vacuum chamber.
DE 100 084 82 A1 discloses an RF plasma source comprising a magnetic field coil arrangement and a unit for extracting a plasma beam, wherein a transverse magnetic field is superposed on an excitation electrode and, for generating a transverse magnetic field, magnetic field coils are arranged around a plasma volume. In that case it is possible to choose between capacitive and inductive plasma excitation, wherein the ion energy can be set in a range of from 10 eV to approximately 1000 eV.
A capacitively coupled plasma source is known from EP 0349556 B1, according to which it is possible to extract a plasma beam for example for the removal and structuring of solid surfaces for the production of surface dopings by particle bombardment or for the production of surface layers. This known plasma source comprises a plasma vessel, which surrounds a plasma space, and also two large-area electrodes, which are connected to a radiofrequency generator via a matching network. The areas of the electrodes are chosen such that virtually the entire radiofrequency voltage is dropped across the extraction electrode. The extraction electrode is arranged in an opening in the pot-shaped plasma vessel. A radiofrequency voltage is applied to the other electrode, which serves as a coupling electrode, wherein the extraction electrode acquires ground potential. In the plasma space, plasma is ignited when the radiofrequency voltage is applied to the excitation electrode and a process gas is fed in the plasma space. The plasma automatically acquires a higher, positive potential relative to the extraction electrode, wherein ions of the plasma are accelerated toward the extraction electrode in contact with the plasma and pass through the extraction electrode. The ion current extracted by the extraction electrode is superposed by an electrode current of identical magnitude that flows with the radiofrequency timing, such that, on average over time, an electrically neutral plasma beam is extracted from the plasma source. RF plasma sources of this type are usually used for ion energies at between 50 eV and 100 eV and in an operating pressure range of between 10−4 mbar and 10−2 mbar. In order to improve the properties of the plasma source, the cited document proposes superposing suitably shaped axial constant magnetic fields on the plasma vessel by means of the plasma vessel being surrounded externally by the use of magnetic field coils in which the plasma vessel is arranged concentrically. In that case, in the regions in which the magnetic field lines run parallel to the walls of the plasma vessel, the diffusion motion of plasma particles to the walls can be restricted, whereby wall losses are greatly reduced and the plasma density can be increased. This in turn serves to increase the ion and electron current density in the extracted plasma beam. Usually, two solenoid coils are arranged around the plasma vessel wherein a particularly effective electron confinement and hence a high plasma density are achieved if currents in opposite directions, that is to say mutually repelling magnetic fields, are generated in the plasma vessel.
WO 2005/008717 discloses a capacitively excited RF plasma source for generating a plasma beam shaped by magnetic fields, wherein an increase in the plasma density and hence operation of the source at relatively low plasma pressures are made possible by means of a homogeneous magnetic field, wherein a set of coils or permanent magnets are provided for generating the magnetic field.
The known inductively and/or capacitively excited RF plasma sources constitute cost-intensive solutions, require a large amount of space owing to the use of the magnetic field coils mentioned and have a complicated construction, such that size scaling for the plasma treatment of large-area substrates, for example as rectangular sources for use in architectural glass coating installations or drum installations, is not very suitable.
For the coating and for the etching of surfaces which can be brought very close to a plasma space, gridless RF plasma sources with a plasma to which magnetic fields are applied are also known. Thus, DE 41 096 19 C1, for example, discloses an RF plasma source comprising two electrodes, of which the first electrode is embodied as a hollow electrode and the second electrode, which is to carry a substrate, is disposed upstream of the cavity of the first electrode. The hollow electrode is surrounded by a dark space shielding and has edges which face in the direction of the second electrode and between which are provided projections that are at the same electrical potential as the first electrode. Between the projections, permanent magnets are provided, by means of which a substrate bias voltage can be set independently of the discharge geometry, the discharge pressure and the radiofrequency power.
DE 102 478 8 A1 furthermore discloses a device for generating plasmas by means of radiofrequency discharges, comprising at least two electrodes, between which a plasma discharge can be maintained, wherein one electrode is embodied as a hollow electrode, a grounded area forms a counterelectrode and that side of the hollow electrode which faces away from the plasma discharge is enclosed by a shielding electrode. A substrate to be coated is arranged between counterelectrode and hollow electrode, such that the substrate itself forms the termination of a space filled with a dense plasma, and provides for a high effectiveness of the plasma treatment. On the outer side of the hollow electrode, in the interspace between hollow electrode and shielding electrode, permanent magnets are fitted, which provide for a magnetic field that leads to an increase in the plasma density in the interior of the hollow electrode.
What is disadvantageous about the gridless sources mentioned is that the area to which the plasma is to be applied has to be brought extremely close to the plasma space and in the process has to serve as a temporary wall of the plasma vessel.
Magnetic fields are also used in magnetron sputtering in order to increase the plasma density and to increase the sputtering rate of a material sputtered from a target, with the same operating pressure. Thus, DE 24 318 32 A discloses a cathode sputtering apparatus (magnetron sputtering apparatus) wherein the magnetic force lines that emerge from an active surface of a cathode and re-enter it run between the emergence and re-entry locations and a tunnel-like region is afforded, in which charged particles are held and in which they can move. In that case, the front side of the cathode, which faces the plasma, can be planar or have a concave or convex curvature. Furthermore, the cathode can have a circular or rectangular form. DE 24 172 88 C2 furthermore discloses a cathode sputtering apparatus wherein a magnet device is arranged in such a way that magnetic field lines emerging from a sputtering area and returning thereto form a discharge region having the form of a self-contained loop, wherein the cathode surface which is to be sputtered and faces the substrate to be coated is planar, the substrate can be moved close to the discharge region parallel to the planar sputtering area across the latter and the magnetic field-generating magnet device is arranged on that side of the cathode which faces away from the planar sputtering area.
In the case of the known cathode sputtering apparatuses, the cathode and an anode assigned thereto are connected in such a way that the cathode acquires a potential below the potential of the anode.